The murine Crry/p65 protein and other Regulators of Complement Activation (RCA) proteins inactivate the complement (C) cascade and appear to protect tissues from C-mediated damage. Nevertheless, it is not known how abnormalities in the function of the RCA affect the immune system in vivo. To investigate their role in health and disease, mice deficient in Crry/p65 have been generated by gene targeting. Crry-/-mice do not survive the embryonal stage. Examination of fetoplacental tissues suggests that activation of C at the fetomaternal interface is causing the fetal loss. Breeding Crry+/- animals to C3-/- (C3-/-Crry-/-) mice rescue this lethal phenotype. In this project further characterization of the role of Crry/p65 in embryonic development and in fetoplacental tolerance will be analyzed by 1) studying the expression of Crry/p65 and other mouse RCA proteins in fetoplacental tissues of Crry+/+, Crry+/-, and Crry-/- mice during different stages of embryonic development; 2) investigating how abnormal Crry/p65 expression and C deposition is affecting the fetomaternal interact. Morphological abnormalities within the fetoplacental unit, and the susceptibility of the tissue to inflammation, cell lysis, and other deleterious consequences of C-deposition will be explored. Rescuing of embryonic lethality using soluble forms of Crry/p65 or transgenic mice expressing Crry/p65 will be attempted. Furthermore, using immunohistochemistry, the mechanism by which C is activated will be determined. Finally, the presence of inflammation or aberrant immune behavior will be monitored. Analysis of the inflammatory reaction and the degree of immune complex mediated injury will be studied by monitoring the response to immune complex challenge using different in vivo models of inflammation. In these experiments, Crry+/+ mice will be compared to Crry+/- mice. In addition, we will compare C3-/- Crry-/- and C3-/-Crry+/+ mice in which the C3 deficiency has been partially corrected by wild type bone marrow transfer. These experiment will help understand the critical role of complement and complement regulatory proteins in health and disease. Also it will also enable us to analyze the role of C and the RCA proteins in fetomaternal tolerance and in the reproductive system.